I am not an expert, but will try to clarify for you the term "numbers matching" which I learned from the guys here. The VIN number on the build sheet will match the VIN number under the windshield, number on the engine block pad (located under the alternator) and the number on the tranny. During the build process at the GM plant each build station had a build sheet pertaining to the vehicle in front of them. It would tell them what parts go on that particular vehicle. Like radio (AM, FM, 8 track), rear end (gear ratio), tranny (auto, 4 speed), engine size, etc. Once that car left a certain build station the workers would discard that build sheet not in the garbage can, but in different places on the car (under the rear bench seat, under the carpet, behind the bucket seat back, inside the door, etc.) You use that build sheet to determine if the car is indeed at "true" SS or a "clone" that someone tried to make look like a true SS by adding SS things to it. You look for a code on the your build sheet that not only has your VIN on it, but also the code "Z25 SS 396 PKG" which is what is on my 396 build sheet or any which are a true SS. People have tried to duplicate and change build sheets so you have to be careful. You can search "build sheet" here and probably see what they look like. If someone is selling a car and does not mention if there is a build sheet then you can assume it's not a numbers matching car. Do your research and learn. I would suggest contacting member Dale McIntosh and buy his books about the Chevelle. They will get you a lot of pics and info. Patrick Glen Nichols is a member expert who will go out and inspect / verify the car you are considering buying. Sorry this is so long, but that is what helped me learn how to not get taken to the cleaners by an untruthful seller.